If you own a boat or love the water, the engine is one of the most important parts of any ship. It’s important to consider when considering how the boat will be used. There are a lot of boat engines and propulsion systems on the market. Spare parts, differences, and key features that distinguish them are discussed below.
The main types of marine engines
It is advisable to know the basics of engines and propulsion systems so you can make an informed choice when selecting a boat or engine. It is appropriate that the dealer or trained technician be able to answer technical questions, but a basic understanding of what options are available could make a significant difference. There are a lot of marine engines and propulsion systems out there, but the four main types are outboard, inboard, stern drive, and jet drive.
- Outboard Engines
An engine, which is the unit that contains the engine, gearbox, and propeller, is also called an outboard motor. This part is mounted to the back of the boat and hangs outside the hull. To drive and steer the boat, an outboard engine is used. It is therefore necessary to shift the whole motor backward and upwards while steering and up and down whilst trimming and towing. Smaller boats can drive the engine manually using a hand tiller, and larger vessels are equipped with steering wheels. During use, the boat's gear and engine propeller case is submerged to give it the ability to fly at a speed of up to 35 km per hour and be very responsive when turning and docking. Outboard motors are useful for fishing, water sports, and recreational boats.
- Inboard Engines
Inboard propulsion is mounted on the ship's hull amidship, the middle of a boat. The propeller's shaft, which runs through the bottom of the boat and exits at the boat's transom, turns the propeller. Steering is controlled by a rudder that controls the flow of water as it moves, enabling the driver to control the vessel with his or her steering wheel. The choice of inboard engines is usually made for boats that should operate on water sports, as they provide a predictable wave that wakeboarders love. They are also applied in ships where more power and torque are required.
Also known as inboard/outboards, sterndrive engines have the features and benefits of both outboard and inboard engines. Sterndrives are equipped with an engine that has been installed inside the hull to provide power. They’re attached through the transom to a drive unit resembling an outboard's lower gearcase portion. This drive unit is the one that drives a boat like an outboard engine. The Stern Drive will turn like an engine of the Outboard, steering the propeller and turning the boat. For applications where more horsepower and torque are required, as well as the ability to maneuver and dock efficiently, stern drives may be useful. In pleasure cruisers, the Sterndrive is very common.
- Jet Drive/Jet Propulsion
The use of jet-drive engines is mostly associated with private watercraft, but many vessels also feature jet-drive propulsion systems. The water used for the propulsion and steering of the boat is utilized by these systems. To make this happen, an engine drives an impeller, which is like a propeller, but sharper. The impeller, which is placed below the ship and pushes water through a tiny hole at the rear of the boat to generate thrust. When a stream of water has changed direction, the boat shall be guided. The jet drive systems are highly effective and work best in water below a certain depth. There's not even a blade of the propeller out there.
Components of Marine Engines (spare parts)
Several thousand spare parts in a marine engine are essential for it to run. In this section, we shall divide the engine into its significant parts and try to explain its purpose and work.
The bottommost part of the engine, which supports the weight of the engine, is the bed plate. Therefore, that part of the engine is loaded to the highest extent. It's placed on a chock, secured to the floor by holding-down bolts that make it stable. The bedplate must be strong enough to withstand the weight of the engine and flexible enough to bend when the hull bends during the hogging and sagging of the vessel. The bedplate is usually made of cast steel and is formed by connecting two longitudinal girders through a series of transverse girders. There are two semicircular openings to support the crankshaft on these transverse girders. Bedplate function:
- Support the engine weight
- Supporting dynamic loading of operating parts
- Get the lube oil and drain it into the tank
- At all times maintain the proper alignment of the crankshaft
The frame appears to be the letter 'A', as the name implies. On the bottom, it rests on the bedplate, and on the top, it supports the cylinder block (or entablature). A frame shall be put up over each of the transversal beams on the bedplate. To improve the sealing, a jointing ingredient shall be applied to both sides of the frame and the bedplate. A frame is secured to the engine by fitted bolts and tie rods.
Fitted bolts connect the bedplate and A-frame whereas the tie bolt secures the entablature, A-frame, and the bedplate with the vessel. The A-frame and the bedplate cavity (between transverse girders) form an enclosed space and isolate each unit of the engine. This enclosed space forms the crankcase for the engine. The entire A frame is cast as a unit with smaller engines. For larger engines, the complete A frame is cast as 2 or 3 separate units and then bolted together. The crosshead and the guide are situated inside the A-Frame. In newer engines, the guides are made in place and cannot be adjusted. A frame function:
- Take your crankshaft in the proper position
- Hold the crosshead and its guide in your hands
- Support the cylinder's block or entablature
- Form the crankcase space
3. Entablature or cylinder block
The entablature rests on top of the A-frame and houses various parts of the engine such as the scavenge space, stuffing boxes, jacket cooling water spaces, and the cylindrical cavity for the cylinder liner. In older engines, the jacket cooling water space would be inside the entablature, but in newer engines, the space is between the jacket and the liner and the jacket fits into the entablature.
The structure of the entablatures is strong enough to withstand the force of combustion. For the entablature, casting iron works best. They are ill-equipped to handle the thrust of an engine that is trying to separate 3 components Bedplate, A-frame, and Entablature. It's a job for tie bolts. The function of the Entablature or Cylinder Block is as follows:
- House the scavenge spaces and the liner cavity
- Support the liner, the cooling water jacket, the cylinder head, the inlet and exhaust valve, and other connected components
The crankshaft is one of the most critical elements of the engine. This part along with the connecting rod converts the reciprocating motion of the engine pistons into the rotational motion of the propeller. The propeller converts this torque into axial thrust and moves the vessel. The crankshaft is subjected to a range of varying loads from the piston, combustion, propeller, and flywheel. It must therefore be designed keeping these cyclic loads in mind. There's a journal, a crank web, and a crank pin in the crankshaft. Generally, it is an alloy of steel, but the characteristics used in the shaft are subject to variation on a case-by-case basis. The characteristics of the crankshaft are made up of elements such as silicon, nickel, vanadium, and chromium. Crankshaft function:
- Use power from electric generating equipment, such as a firing cylinder, and transmit it to the energy units used by propellers, motors, engines, flywheels, or pumps for suction stroke operation
- Convert linear motion to rotational motion
- Distribute the axial and rotation forces generated by the movement of the ship to the hull by means of axial and rotation bearings.
5. Connecting rod
The connecting rod (or con rod) connects to the crankshaft on one end and the crosshead on the other. Once the piston is going up and down, it'll move the crosshead with itself which will lead to a similar motion of the Connecting Rod. Using this motion, the connecting rod moves the steering pin in a circular direction and so does the crankshaft. During the operations, the rod undergoes tension, compression, bend, and bowing forces. On the piston side, it is called a small end, and on the crank pin side, it's called a large end. The link rod shall have bearings on both sides to ensure that the operation does not result in any damage. As with the crankshaft, the connecting rod is subject to constant cyclic loading. Connecting rod function:
- convert the reciprocating motion of the crosshead to the rotation motion of the crankshaft
- The oil shall be supplied by the crosshead to the crankpin through the bores for cooling and lubrication.
The crosshead is a rectangle, which connects the engine's piston and connecting rod. There's a circular cross-head pin in the center, which is called Cross Head Journal or Crosshead Pin. This pin shall be fitted with an eye of the connecting rod at its small end. There's a provision in the crosshead bear cap that allows the piston rod to connect by fasteners and bolts to the crosshead journal. The lubrication oil for the crosshead pin bearing is supplied through a telescopic pipe. The oil travels through the drilled bores to the piston and to the crank pin for lubrication. The crosshead's got a pair of guide shoes across either side. The shoes' curved faces are covered in white metal and they move on the rails known as crosshead guides. Crosshead function:
- By transferring it to the engine structure instead of the piston, you can eliminate the side load on the piston and the cylinder liner.
- Supply lubricating oil to the crankpin bearing and the piston
- Allow the connecting rods outside of the cylinder to move freely
The piston is a set of composite parts that transfers gas pressure to mechanical forces in the engine. It's attached to the engine's cylinder and transfers the mechanical force to the piston rod, the two-stroke engine, or the four-stroke engine connecting rod. The crowns and skirt of the pistons consist of two distinct parts. On each side of the skirt, 16 or more bolts connect and secure them to a locking wire. By the use of another set of bolts, the piston rod is secured to the inside of the crown. High thermal and shock loads are applied to the piston. Usually, heat is used to treat them. Piston function:
- Put the power from the engine inside of the cylinder to the crankshaft, via a crosshead and tie rod arrangement.
- With a vacuum stroke, the compressed air-fuel mixture
- Make sure the combustion chamber is sealed to prevent hot gases from blowing up
8. Cylinder liner
Cylinder liners are placed inside the engine's cylinder, which is to be used as a combustion chamber. It is directly exposed to the compressed air-fuel mixture during the suction stroke and the combustion pressure during the power stroke. Cylinder liner function:
- For the combustion process, heat dissipation shall be performed
- Form a sliding surface that helps the piston's movement
- Make sure that the combustion chamber is well sealed. As erosion develops and there is an increase in the gap between the piston rings and the lining, the sealing capacity decreases over time.
9. Cylinder's head or cylinder cover
The cylinder head is the topmost part of the engine structure and houses various monitoring instruments and mountings such as the fuel valve (or fuel injector), starting air valve, cylinder relief valve, indicator valve, exhaust valve, etc. It also has cavities for cooling water circulation. From the top, the cylinder head shall cover the sealing of the combustion chamber. Cylinder head function:
- Seal the combustion chamber
- Transfer the combustion forces to the engine structure
- Act as a platform allowing for the start of air and fuel to be used in the system
- Establish a passage from the liner to the exhaust valve for cooling water
10. Exhaust valve
Each engine has an exhaust valve that attaches to the central bore of a cylinder head. The exhaust valve shall open at the end of combustion and remove any exhaust gases from the engine compartment. From the exhaust valves, the gases are first transferred to the manifold, then to heat recovery systems (turbocharger, economizer) and finally let out through the ship’s funnel. Exhaust valve function:
- For the set time extraction of exhaust gases must be performed as soon as possible.
- Ensure effective sealing when closed in order to avoid any leakage of compressed air or tailpipe gases
- Transfer gases into the manifold so that they can be used again
The main engine camshaft is a rotating part with a series of fixed cams at different angles. The camshaft uses chains or gears to generate power through the engine's crankshaft. It spins cams with it when it's rotating. Camshaft function:
- Convert the rotational motion of the crankshaft to the reciprocating motion of cam rollers
- Operate inlet and exhaust valves with four-stroke engines and only exhaust valve with two-stroke engines
- Use the fuel injection pumps and the starting air distributor. Nevertheless, on modern vessels, the common use of rail injection systems for fuel delivery is beginning to take place.
A forced induction device, consisting of an exhaust gas generator and a centrifugal pressurizer, is the primary engine turbocharger. In order to allow for the supply of compressed fresh air into the combustion chambers, both devices are set up in such a way that it is possible. In order to increase the engine's power output, an increased amount of oxygen is intended to be supplied by air. There are usually two turbochargers on a large marine engine. Turbocharger function:
- Increase the amount of fresh air for combustion
- In order to increase engine efficiency use the residual energy of exhaust gases